Use of high power wireless communication standards, such as Global System for Mobile Communications (GSM), Enhanced Data Rates for GSM Evolution (EDGE), General Packet Radio Service (GPRS), Terrestrial Trunked Radio (TETRA), and Citizens Band (CB), has increased significantly. These standards may be incorporated in communication systems that are co-located with other RF systems susceptible to signal interference, such as television (TV) systems, operating in the very high frequency (VHF) and ultra high frequency (UHF) television bands. The proximity of base stations, handsets and other receivers/transmitters, particularly in densely populated urban areas, may result in television receivers being exposed to high interfering signal strengths from the co-located communications systems. For example, these interfering signal strengths may be sufficient to produce interfering signals of 100 dBμV or more at the television aerial antenna.
Accordingly, reception by broadband receivers, such as radio frequency (RF) television receivers, may be blocked during transmission of various co-located system device operating in the same bandwidths. As a result, reception quality of incoming television signals may be deemed unacceptable.
In order to prevent inference from the co-located systems, notch filters may be added to the RF side of the receiver to reject the co-located system signals.
For example, notch filters may be included in tuner 120 of television receiver 100, shown in the block diagram of FIG. 1. The television receiver also includes antenna 110, channel decoder 130 and source decoder 140 for receiving and decoding signals, which are displayed, for example, on monitor 150. The antenna 110 receives television band signals, as well as out-of-television-band signals i, i+1, i+2, . . . i+x from co-located systems. The out-of-television-band signals i, i+1, i+2, . . . i+x are within the RF spectrum and may potentially interfere with the television signals, unless properly filtered. Even though the probability of all co-located systems being active at the same time is typically low, all of the notch filters are cascaded in order to minimize the possibility of interference from one or all of the co-located systems, and thus to provide a robust solution. Consequently, the television receiver includes as many notch filters as there are co-located systems. Each notch filter introduces associated implementation loss, resulting in a corresponding loss of sensitivity for the receiver. For example, the receiver 100 would include three notch filters, respectively tuned to frequencies corresponding to signals i, i+1 and i+2. Thus, the sensitivity loss increases with the number of potentially interfering co-located systems.
In one aspect of the invention, a method is provided for adaptive filtering. The method includes receiving multiple signals in a predetermined radio frequency (RF) spectrum, the signals including a desired signal and multiple potentially interfering signals; down-converting a first signal of the potentially interfering signals to a baseband signal; and determining power of the baseband signal. It is determined whether the power exceeds a threshold power. When the power does exceed the threshold, a first notch filter corresponding to a frequency of the first signal is activated.
In another aspect of the invention, a system is provided for adaptively filtering signals in a predetermined spectrum of an RF receiver, the predetermined spectrum including a desired signal. The system includes multiple selectively activated notch filters, a power detector, a local oscillator (LO) generator, a mixer and a processor. The selectively activated notch filters are configured to filter corresponding frequencies of potentially interfering signals in the predetermined spectrum. The power detector is configured to detect an aggregate power of received signals in the predetermined spectrum and to determine whether the aggregate power exceeds a predetermined maximum power. The LO generator is configured to generate LO frequencies corresponding to the frequencies of the notch filters, the LO generator generating a first LO frequency corresponding to a first notch filter of the multiple notch filters when the aggregate power exceeds the predetermined maximum power. The mixer is configured to mix the received signals with the first LO frequency to down-convert the received signals to a baseband frequency. The processor is configured to determine whether power of the baseband signal exceeds a threshold power and, when the power exceeds the threshold power, to activate the first notch filter.
In another aspect of the invention, a system is provided for adaptively filtering signals in a predetermined spectrum of an RF receiver, the predetermined spectrum including a desired signal. The system includes multiple selectively activated notch filters, a power detector, a data path, a filter and a processor. The selectively activated notch filters are configured to filter corresponding frequencies of potentially interfering signals in the predetermined spectrum. The power detector is configured to detect an aggregate power of received signals in the predetermined spectrum and to determine whether the aggregate power exceeds a predetermined maximum power. The data path is configured to demodulate the desired signal regardless of the aggregate power. The filter adaptation path is configured to demodulate at least one of the potentially interfering signals to a baseband signal substantially simultaneously with the data path demodulating the desired signal when the aggregate power exceeds the predetermined maximum power. The processor is configured to determine whether power of the baseband signal exceeds a threshold power and, when the power exceeds the threshold power, to selectively activate a first notch filter of the notch filters corresponding to the demodulated potentially interfering signal.